Fluid cylinders (e.g. linear hydraulic actuators) generally include a piston moveable within a hollow cylinder The piston is coupled to a piston rod and defines a chamber within the interior of the cylinder. By pumping pressurized hydraulic fluid such as oil into and out of the interior chamber, the piston is moved within the cylinder. The piston rod is therefore moved relative to the cylinder, extending or retracting to do mechanical work. The cylinder typically has a constant cross-sectional area and the volume of oil pumped into the chamber is directly proportional to the force applied to the piston and inversely proportional to the speed of the piston. A cylinder with a relatively large cross-section will therefore actuate more slowly, but with a larger force. A cylinder with a relatively small cross-section will actuate more quickly, but with a smaller force.
Generally, the capacities of conventional fluid cylinders compromise both force and speed. Conventional fluid cylinders are typically sized to produce enough force under a system working pressure to meet high load conditions, but also to produce an acceptable movement speed at maximum system flow. To achieve maximum actuation speed in an unloaded fluid cylinder, the flow rate of the fluid is increased, such as with a variable flow pump or by increasing the speed of an engine driving a fixed flow pump. A fixed flow pump typically has a volume that is proportional to the shaft speed, such that when there is little demand in the system the excess oil is inefficiently bypassed back to a sump or storage tank.
Accordingly, it would be advantageous to provide an improved fluid cylinder that is capable of multiple output forces and speeds for use with a fluid source providing a single input pressure.